MacOS – Can’t reformat SD card (Yosemite)

There might be a misconception somewhere in what you say. You can run Disk Utility from the original system disc WITHOUT re-installing OS X.

I ran into a similar problem recently, on which running Disk Utility from any of the existing partitions would not work. I then tried running Disk Utility from a restore disk I had copied to an external USB-drive and to my surprise it was not successful either. Ultimately, the only way I was able to get Disk Utility to re-partition my drive was when I executed it from the original system CD that shipped with my Mac.

Again, you can launch Disk Utility from the setup start up screen by selecting it from one of the menus. You do not require to re-install OS X. Hope this helps.

Lion has a hidden partition which contains recovery information. Shrinking the partition, according to an Apple employee, is not supported. He hooked up my MBP to an external hard drive and wiped and reinstalled Lion in order to fix things.

In more detail, what happens is this: When you try and resize the main partition, it doesn't include the recovery partition. When you hit "Apply Changes," it does a checksum to make sure that nothing's accessing the drive. It then gets started, doing another checksum, and discovers that something's changed, breaking the previous checksum.